Processes for preparing thermoplastic resin expanded bodies by injection molding includes a process wherein carbon dioxide, nitrogen gas or like nonreactive gas serving as a blowing agent is incorporated into the molten resin and then the resin is injected into a mold for expansion.
Such expanding nonreactive gas is incorporated into the molten resin by 1) a method wherein the nonreactive gas is incorporated into the material resin in a solid state under a high pressure before placing the resin into a cylinder, or by 2) a method wherein the nonreactive gas is incorporated into the resin as melted within a cylinder as disclosed in the specification of U.S. Pat. No. 5,158,986.
However, although capable of affording a thermoplastic resin expanded body, the former method of incorporating the nonreactive gas into the solid material resin requires several tens of hours to saturate the resin with the gas for incorporation, for example, because there is a need to incorporate the gas into the resin within a pressure-resistant chamber, with the supply of resin to the molding machine interrupted, in order to completely incorporate the gas into the resin since the gas and the resin are low in affinity for each other. The method is therefore difficult to practice on an industrial scale.
On the other hand, with the latter method wherein the nonreactive gas is incorporated into the molten resin in the cylinder, the resin as in the form of pellets and supplied to the cylinder from a feed inlet is melted by the rotation of a screw within the cylinder, and carbon dioxide gas is supplied from a gas inlet in a portion of the cylinder while an amount of molten resin is being measured out by a measuring portion at the forward end of the cylinder, with the screw retracted. Since the resin within the cylinder is subjected to a high pressure during measurement, it is difficult to inject the gas into the molten resin unless the gas to be supplied is given a higher temperature than the molten resin. In the case where a gas of high pressure is used, the apparatus itself must have a pressure-resistant structure. This entails an increased production cost.
In view of the foregoing problems of the prior art, an object of the present invention is to provide an injection molding process for preparing thermoplastic resin molded articles and an injection molding apparatus for use in practicing this process, the process and the apparatus being adapted to supply a blowing agent, especially a nonreactive gas such as carbon dioxide or nitrogen gas, to a thermoplastic resin continuously at a relatively low pressure with good stability for the incorporation of the agent or gas into the resin.